The present invention relates to methods and apparatus for collecting heat, such as solar radiant, energy by heating fluid, such as water. Particularly, the invention concerns a radiation collection system which comprises radiation-collection element and which may be a part of a water heating system in a building or facility. More particularly, the present invention relates to the open-loop solar energy powered systems providing hot water for domestic or other use simultaneously using it as a heat-collection and heat-transfer medium.
In solar collector panel (U.S. Pat. No. 4,258,701 to Buckley, publ. 1981 Mar. 31), which may be used in conventional open-loop solar water heating system, in its heat transmission module, a natural-circulation, solar collector, storage tank, insulation means for insulating and separating the storage tank from collector, and other unrelated parts are fabricated as a unitary structure capable of bearing load. Said panel solar collector is mounted with an inclination of a top solar-radiated surface to better receive radiant solar energy. Storage spread over wall surface acts as a thermal barrier to heat. When the circulating water temperature is low, the additional heating of the whole water storage tank is possible. However, with increasing the temperature of circulating water, sufficiently decreases the heat transfer through the collector wall and, correspondingly, the solar collection efficiency (at instantaneous solar radiation 850 W/m2 for the circulating water temperature 50-70 degrees higher than the ambient temperature, the real efficiency of the collectors with advanced black-chrome glazed coatings is about 30-45%). The limited solar collection efficiency determines the lower balance temperature and correspondingly limited water-heating capacity. Due to the low velocity of line-pressure water circulation, the minerals from the water precipitate, clogging the narrow channels of the collector. The panel solar collectors of open-loop systems are susceptible to freezing in the cold-winter climate.
Close-loop indirect circulation system described in U.S. Pat. No. 4,421,100 to Yu (publ. 1983 Dec. 20) having the thermosyphon heat pipe water heating appliance uses latent heat of heat-transfer liquid in the separate low-pressure solar collector circuit connected to the tank. The collector is operated to vaporize the first heat-transfer fluid (Freon) to be condensed in upper duct so that the latent heat is transferred to the second low-pressure fluid in the tank with a ducting for passing to the third fluid (line-pressure water). The close-loop systems are complicated by having separate fluid circuits with energy losses at heat transfer from one circuit to another and sufficiently higher required circulating temperature of the first fluid that sufficiently decreases efficiency of solar collection. The evacuated-tube collectors using the heat-pipe principle, while more efficient than the flat-plate collectors at higher temperatures, are sufficiently more complex and expensive.
Close-loop indirect circulation system described in U.S. Pat. No. 6,119,682 to Hazan (publ. 2000 Sep. 19) having integrated flat solar radiation absorber panel and insulated fluid storage tank operates by thermosyphonic circulation of the heat-transfer liquid (demineralized water with added antifreeze substance, corrosion inhibitor, etc.) through the collector channels and conduits to the tank; an extended-length heat-exchange piping circuit with the line-pressure water; storage tank housing having a cold-water inlet port and hot-water outlet port and an electric water immersion heater. The two-circuit system has low heat-transfer and solar radiation collection efficiency and correspondingly low temperature of the water in the tank until the electric water immersion heater is used.
Modular solar radiation collector described in U.S. Pat. No. 4,111,187 to Wiegand (publ. 1978 Sep. 5) comprises spray means extending into the cavity formed by an upper radiation absorption surface and a lower drainage surface. Fluid is sprayed through nozzles spaced along water conduits situated in the internal cavity against the underside of said upper radiation absorption surface and drained through outlet means adjacent to the lower end cap. The spaced nozzle positioning against the underside of the upper radiation absorption surface creates its excessive moisturizing, which at comparatively week level of radiation, such as solar one, doesn't allow the water evaporating with its consequent condensation and latent heat utilization. Thus, due to low intensity heat transfer from said underside surface, the collector has low water-heating efficiency.
In the above prior art systems, the solar energy collection efficiency sufficiently decreases at high circulation temperature. Devices used, particularly, in the open-loop systems are characterized by low productivity and reliability, remarkably in the cold-winter climate. For increasing the heat-abstraction efficiency, the pulsating devices, as in the prior art apparatus described below, and corresponding methods may be used.
A boiling-film heat-transfer device containing a vertical copper tube with the heat transfer wall having an evaporation surface with the indentation of artificial pores of depth from 0.2 to 0.3 mm of the same diameter leads to a liquid film boiling with heat transfer enhancement up to 2.2-3.4 times comparatively with pool boiling (according to B. Haase, Der Warmeubergang am sidenden Rieselfilm, Chem. Tech., 22H, 5. (Mai, 1970) 283-287—see I. I. Gogonin, Heat transfer in boiling liquid in a film moving under gravity, Journal of Engineering Physics and Thermophysics, July-August, 2010, p. 876-881 (83-4), Ref. 4). The high-efficient film boiling of liquid is associated with restricting the vapor flow from the wall by the entrainment of a portion of moisturizing liquid from a film after vapor bubble collapse, which is technically hardly achievable because requires a very accurate tailoring of the film conditions.
A heat accumulating device in Japan Patent 56138645 to Wada Takahiro (publ. 1981 Oct. 29) comprises a solar radiation collector and a heat exchanger connected with an indoor air conditioner and provided with an atomizing device fitted with an ultrasonic-wave oscillating element, which is purposed to enable condensation of solution by means of a low-temperature heat source by applying ultrasonic waves to diluted solution to make the same atomized and by heating the same later to be vaporized in the device wherein heat radiation and heat accumulation are performed by repeating dilution and condensation of the solution. By atomizing the diluted solution by means of the sufficiently powerful ultrasonic-wave oscillating element, the surface area of the solution is increased and the vaporization rate is accelerated, while the size of the droplet of liquid and the atomization rate can be controlled by varying the frequency and output of the ultrasonic waves. However using powerful ultrasonic device to sufficiently improve in such a way the heat transfer compromises the device reliability and health safety and requires additional energy consumption.
A pulsator in U.S. Pat. No. 5,314,116 to Krauth and Brunnengraeber (publ. 1994 May 24) comprises a casing having an expansible elastic tube defining partially an expansible chamber and adapted for control of an intermittent opening in said chamber by closing pressure-controlled valve in contracting position in at least near line contact, an inlet with flow control communicated to said expansible chamber. When the pressure in the chamber exceeds a predetermined level, the valve will open to intermittently discharge pulses of liquid. The normally closed valve preferably comprises an annular contact surface to induce near-instantaneous opening during a pulsating cycle. The elastic tube device is not reliable and, due to rheological properties of the elastic tube material, not applicable for high-frequency low-volume pulse flow with sufficient momentum; the bulky and complex design of the elastic tube is intended for irrigation but doesn't match into small space of the internal cavity for the solar-radiation collection purposes. Due to the above-mentioned scale factor and elastic characteristics varying with the temperature change, the elastic tube device doesn't provide reliably intermittent pulsating flow of the heated water.
A pulsating device for converting pressurized low-rate continuous flow to high-rate intermittent pulsating flow in U.S. Pat. No. 5,507,436 to Ruttenberg (publ. 1996 Apr. 16), which may be used for increasing the heat abstraction efficiency, comprises a somewhat expandable chamber having flow inlet with controlled flow rate and outlet provided with a small-diameter tube having a deflector and nozzle at an opposite end and a normally-closed pressure-responsive valve having a preset pressure response to create a water hammer effect. The valve contains expandable elastic sleeve and openings, which create a pumping effect to admix air with the affluent stream, ejecting fluid from said expandable chamber through said outlet in a high rate causing the volume and pressure of the liquid in said chamber to decrease and to close said outlet and openings, thereby to complete a cycle of an intermittent pulsating flow. However, it's hard to provide reliable characteristics of said elastic sleeve, which controls said preset pressure level and the pulse characteristics applicable for high-frequency low-volume high-velocity pulsating flow. The elastic properties vary at wide temperature range and don't provide the device reliability with durable low-varying pulse characteristics. While the spray means connected to this pulsator is claimed to be capable to spray liquids to a large designated area, the air-enriched atomized flow has insufficient momentum for creating required distinctive velocity plume in the space of internal cavity of solar radiation collector and would decrease the heat-abstraction efficiency.
Device for water desalination described in U.S. Pat. No. 5,650,050 to Kaufmann (publ. 1997 Jul. 22), including the elements of the solar heat abstraction, comprises an element arranged in an inclined position and having a top solar-radiated surface and a bottom evaporation surface within container having an inlet for water to be desalted and outlet for water vaporized or evaporated at said element; condenser coupled to said outlet arranged in an upper area of said container; rinsing means connected to water feed pipe containing nozzles arranged to spray water in a distributed manner against said bottom evaporation surface. The device contains a draining pipe coupled to a lower area of said container; openings arranged there through which outside air can be drawn in; and a suction device coupled to the said outlet in an upper area. In the method described in U.S. Pat. No. 5,650,050 to Kaufmann, the water is sprayed against the underside of the element; in order to evacuate the evaporating water avoiding its condensation within the element and draining with downward-running part of the sprayed water, the outside air motion is provided by a fan situated at lower area and suction device in the upper area, which sufficiently complicates the device functioning and decrease the heat transfer intensity and efficiency. In order to provide necessary heating of the surface by solar radiation, the water sprayed preferably for period of only a few seconds at predetermined by controlled valve intervals up to 90 minutes. To clean the element rinsing away deposited salt, the water may be sprayed, for initial cooling, every 0.5 to 1.5 hours for 0.67 to 2 minutes or, at low temperature, every 1 to 1.5 minutes for 1 to 6 seconds, preferably 2 to 4 s, which at least 1-minute-long spraying intervals are determined by the time necessary for the evaporating water evacuation during the solar-radiation powered evaporation process. Such relatively short spraying periods with intensive heat abstraction require powerful devices for vapor removal; such method and device are low-efficient for the water-heating purposes.